Power device comprising a power transistor and a Schottky diode integrated with the power transistor is often required in power management circuits. FIG. 1A illustrates a simple power conversion circuit 100 configured on the basis of a power device comprising a power transistor 101 and a Schottky diode 103 connected in series to convert an input voltage Vin to an output voltage Vo. The power transistor 101 comprises, for example, a junction field effect transistor (JFET) having a gate (G) connected to ground. Thus a complicated control circuit is not required. However, the JFET is a normally-ON device that conducts current in either direction from drain (D) to source (S) or from source (S) to drain (D). Therefore, without the Schottky diode 103, when the output voltage Vo is higher than the input voltage Vin, a current flows reversely from Vo to Vin. The Schottky diode 103 is provided for blocking this reverse current flow.
FIG. 1B shows a diagram illustrating a current (IIN) through Schottky diode 103 and JFET 101 as a function of Vin, assuming the gate and the source of JFET 101 are grounded. When the input voltage Vin is higher than a forward conduction voltage VF of the Schottky diode 103, the current IIN flows from the input Vin to the output Vo through the forward biased Schottky diode 103 and the normally-ON JFET101. As Vin increases further, the JFET 101 may “pinch off” at a certain voltage (VP) such that the current IIN remains substantially constant for higher values of Vin. In a typical application, the output voltage Vo is not grounded, but rather is used to power a next-stage circuit. In such configuration, Vo rises with increasing Vin up to VP, and then remains relatively constant. Thus, JFET 101 passes a current from Vin to the next-stage circuit while blocks high voltages (voltages higher than VP) which may damage the next-stage circuit. When Vin reaches a forward breakdown voltage (VBF) of the JFET 101, a large current flows from Vin to the gate of JFET 101 and/or to Vo. When Vin is negative with respect to Vo, the current IIN turns to be an off-state leakage current (TOFF) flowing from Vo to Vin. When the negative Vin voltage exceeds a breakdown voltage (VBR) of the Schottky diode 103, a high current flows from Vo to Vin.
FIG. 2 illustrates a sectional view of a prior art power device 200 integrating a JFET 202 and a Schottky diode 204. The power device 200 has a p-type substrate 206. An n-type well region 208 is formed on the p-type substrate 206. The JFET 202 and the Schottky diode 204 share the same p-type substrate 206 and the same n-type well region 208. A p-type doped region 210 is provided in the n-type well region 208 to form a gate region of the JFET 202 and a p+ heavily doped region 212 is formed in the p-type doped region 210 to function as an ohmic contact for the gate region 210. The n-type well region 208 at the left side of the gate region 210 forms a drain region of the JFET 202 and the n-type well region 208 at the right side of the gate region 210 forms a source region of the JFET 202. An n+ heavily doped region 214 is provided in the n-well region 208 at the right side of the gate region 210 to form an ohmic contact of the source region. A drain metal 216, a gate metal 218 and a source metal 220 functioning respectively as a drain electrode D, a gate electrode G and a source electrode S of the combined power device 200 are formed to respectively contact the drain region, the p+ heavily doped region 212 and the n+ heavily doped region 214.
The Schottky diode 204 comprises a cathode sharing the n-type well region 208 and an anode sharing the drain metal 216. The Schottky diode 204 further comprises p+ heavily doped regions 222 provided on both sides of Schottky diode 204. P+ regions 222 are used to form a merged PN Schottky (MPS) diode, which reduces a reverse leakage current of the Schottky diode 204. Without the p+ heavily doped region 222, the reverse leakage current of the Schottky diode 204 may be unacceptably high.
However, the p+ heavily doped region 222 may cause a problem when the power device 200 is in ON state and the Schottky diode 204 is forward biased. In fact, there exists a parasitic bipolar junction transistor (BJT) which uses the p+ heavily doped region 222, the n-type well region 208 and the p-type substrate 206 as an emitter, a base and a collector, respectively. At high forward current, the junction between the p+ heavily doped region 222 and the n-type well region 208 may be forward biased, causing the parasitic bipolar transistor to turn ON. In this case, carriers may be injected into the substrate 206 and disturb the operation of other circuits that are integrated with the power device 200, which is undesirable.